构造生烃理论及其在页岩气上的应用

Tectonic generation theory and its use in shale gas

生烃是地层有机质生成油气的化学平衡。由于油气的密度低于干酪根,它是典型的体积增大化学反应。与实验室开放系统不同,地层有机质生烃反应能等于活化能加排烃能。成熟阶段的地层比较致密,排烃能较大,与开放系统相比,形成了欠生烃。构造运动形成裂隙网,大大降低地层排烃能,使欠生烃的有机质短时间集中生烃,笔者称之为构造生烃。成熟地层通常较致密,排烃能较高,较多欠生烃有机质成为页岩气的物质基础。致密地层在过成熟条件下还有大量欠生烃有机质。经典生烃理论认为Ro大于2.0就基本不生烃,而许多Ro达到3.0,个别甚至4.0的页岩气发现,证明欠生烃的存在。分子越小,排烃能越低,相对致密地层生气的反应能通常最低,更多的有机质形成了页岩气,页岩气资源潜力巨大。页岩气的开采速度比地层自然排烃速度高出多个数量级,天然气排出最快的,最有利于生成天然气的化学平衡。勘探开发实践表明页岩含气量与其TOC成正比,笔者认为这正好预示着页岩中存在游离气、吸附气和有机质的化学平衡。游离气压降低,吸附气就会解吸附;吸附气解吸附,有机质就会生烃。有些过成熟岩石存在未—低成熟度Tmax值。这样低的Tmax值预示着,有机质能够在地层被压裂后随着排烃能的降低而满足反应条件,而成为潜在资源。页岩气的开采与普通气层相比更复杂、更漫长、更巨大。

Hydrocarbon generation is chemical balance and organic matter in rock generates oil or gas.Since the density of oil and gas is lower than kerogen,the reaction is a kind of classic chemical reaction with volume increase.Different from open system in laboratory,hydrocarbon generation reaction under earth equals activation energy plus draining hydrocarbon energy.The density of mature rock is quite dense and its draining hydrocarbon energy is quite high comparing with open system,it makes under generation.Structure movement forms fracture net,and draining hydrocarbon energy is decreasing greatly,which makes under generation organic matter generate a lot of oil and gas in short time and it is named as Tectonic Generation by the author.Mature rock is more compact and its draining hydrocarbon energy is higher generally.More under generation organic matter become material base of shale gas.Over mature compact rock contains plenty of organic matter.Classic generation theory insists that generation ends basically while Ro is over 2.0.But plenty of shale gas found in formation with Ro 3.0,even with Ro 4.0,it proves the existing of under generation.The smaller the molecule is,the lower the draining hydrocarbon energy is.Much organic matters in the rock generate shale gas,so potential of shale gas is huge.Since the developing rate of shale gas is several orders of magnitude higher than draining hydrocarbon rate in nature,gas draining is fast and it benefits the chemical balance to generating gas.Exploration proves that shale gas quantity in direct proportion to TOC,which shows that free gas,absorbed gas and organic matters forms chemical balance in shale.While free gas pressure increases,absorbed gas desorbs,and organic matter generates gas.Some over mature rock have under-low mature Tmax.The low mature Tmax shows that organic matters become potential resources after fracturing and draining hydrocarbon energy decreases to meet the reaction condition.Shale gas development is much more complex than ordinary gas.